Parkinson's disease (PD) is a progressive neurodegenerative movement disorder characterized by selective loss of dopaminergic neurons, and the presence of intracellular protein aggregates termed Lewy bodies. The pathogenesis of PD remains incompletely understood, but it appears to involve both genetic susceptibility and environmental factors. Five major genetic causes have been identified, including mutations in alpha-synuclein, parkin, PINK1, DJ-1 and LRRK2, and these mutations have provided models for study of the pathogenesis of genetic PD, with implications for sporadic PD. Mutations in LRRK2 (or dardarin) have been recently identified to cause autosomal dominant late onset, clinically typical PD with pleomorphic pathology. LRRK2 is a large complex protein, and contains domains that suggest roles for protein interactions, kinase activity, and Ras-related signaling. We have extensive experience in studying cell toxicity, signaling pathways, and protein interactions in neurodegenerative diseases. Our previous studies of PD have elucidated the role of alpha-synuclein toxicity, and protein interactions with synphilin-1 and parkin in the formation of Lewy body-like aggregates. We now propose to use similar methods to study LRRK2. We have evidence that LRRK2 interacts with other PD-related gene products and have promising data indicating that mutant LRRK2 can cause direct neuronal toxicity, providing a unique model for studying pathogenesis of LRRK2-related PD. In Specific Aim 1, we will study LRRK2 cellular expression and toxicity, and determine the signaling pathways by which LRRK2 induces neuronal degeneration. We will conduct cell transfection studies using wild type and mutant LRRK2, examine the LRRK2 distribution patterns, and confirm the LRRK2 toxicity. We will generate stable PC12 cell lines which inducibly express either wild type or mutant LRRK2 to study the molecular mechanisms underlying LRRK2 toxicity. In Specific Aim 2, we will test the functional effects of LRRK2 interaction with synphilin-1 and parkin on the pathways of cell toxicity and cellular aggregation using the cell models described in Aim 1. These studies will elucidate the functional role of LRRK2 in cell toxicity and the functional consequences of its interaction with synphilin-1 and parkin in cell culture, will help elucidate the pathogenesis of LRRK2-associated PD, and will identify potential therapeutic targets for intervention in PD.